This application claims benefit of Japanese Application No. 2001-283664 filed in Japan on Sep. 18, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lens barrel of a camera or the like constructed in such a manner that a plurality of lens frames each holding a photographic lens group are individually movable along an optical axis.
2. Related Art Statement
In recent years, in widespread compact zoom cameras, a zoom lens barrel has to advance or retract so as to cover a photographic range from a wide angle position to a telephoto position. The following zoom lens barrel is put into practical use. In order to further reduce a size of the zoom lens barrel while being carried, the zoom lens barrel is movable from a photographic position such as the wide angle position or the telephoto position to a collapsing position in which the zoom lens barrel is received in a camera body. Furthermore, in recent years, higher zoom magnification has been desired. Accordingly, a difference between the length of the lens barrel at the wide angle position or the collapsing position and the length of the lens barrel at the telephoto position is becoming larger. For the purpose of coping with this situation, serving as the structure of a lens frame assembly which can extend longer, a zoom lens barrel having many lens frames, namely, having a multistage lens frame structure, is needed.
In this kind of zoom lens barrel, generally, a plurality of lens frames holing photographic lens groups are individually movable along an optical axis. Serving as the zoom lens barrel having such a multistage lens frame structure, for example, there is a four-stage zoom lens barrel constructed by combining two single annular frames with two double frames each having a double structure including a frame member having cams or a helicoid and a linearly-movable frame having a straight guide. The zoom lens barrel comprises a first-stage stationary frame fixed to and supported by a camera body, a second-stage rotary frame and a movable frame in the rotary frame, a third-stage cam frame and a float key in the cam frame, and a fourth-stage first lens frame. The zoom lens barrel further comprises a second lens frame and a third lens frame which are received in the cam frame, and an advancing gear.
The first lens frame, the second lens frame, and the third lens frame hold a first lens group, a second lens group, and a third lens group along a lens optical axis of the present lens barrel, respectively. Each lens group comprises a plurality of photographic lens groups.
In the zoom lens barrel having the above configuration, a rotating force is transmitted from the advancing gear to the rotary frame, which is rotatably supported in the stationary frame. Thus, the movable frame is moved forward relative to the rotary frame. Then, the cam frame, which is engaged with through-cams of the movable frame and straight grooves of the rotary frame, is also advanced in association with the above forward movement. Consequently, the first lens frame engaged with a helicoid in the cam frame, the second lens group engaged with at least one cam groove in the cam frame, and the third lens group engaged with the first lens frame are advanced together with the float key in the cam frame. In this instance, in some cases, the first lens frame and the third lens frame are moved together during the zooming operation in order to simplify the mechanism and ensure precision.
In this type of zoom lens barrel, in some cases, in order to accomplish smooth relative movement of the first lens frame and the second lens frame with respect to the cam frame in the direction of the optical axis, helicoid engagement or engagement through cams is used between the frame members. FIG. 6 shows a conventional zoom lens barrel of this type.
FIG. 6 shows a cam frame mounted on the conventional zoom lens barrel and is a development view of the cam frame having cams and a helicoid on the inner surface thereof.
As shown in FIG. 6, on an inner surface 50b of a cam frame 50, a female helicoid 51 and second-lens-group cam grooves 50c are formed.
The female helicoid 51 is engaged with a male helicoid formed on the outer surface of a first lens frame at the rear edge on the side of an image, resulting in forward movement of the first lens frame in the direction toward an object.
The second-lens-group cam grooves 50c are formed on the inner surface 50b in accordance with the number of second-lens-group pins on the outer surface of the second lens frame which is movably fitted into the first lens frame. Generally, the three second-lens-group cam grooves 50c are formed so as to be compatible with the second lens frame having three second-lens-group pins. The second-lens-group cam grooves 50c are engaged with the respective three second-lens-group pins, thereby moving the second lens frame in the direction toward the object in the first lens frame. In the diagram, reference symbol L1 in each second-lens-group cam groove 50c denotes a position of each second-lens-group pin for a second lens group when the present zoom lens barrel is collapsed; L2 a position of each second-lens-group pin when the present zoom lens barrel is located at a wide-angle end position (wide angle mode); and L3 a position of each second-lens-group pin when the present zoom lens barrel is located at a telephoto end position (telephoto mode).
As mentioned above, in the case of the cam frame having the cam grooves and the female helicoid on the inner surface, since the cam grooves are formed in an area where the female helicoid is formed, each cam groove has to be formed on the surface of the root of the female helicoid. In other words, the female helicoid and the cam groove are formed so as not to overlap each other longitudinally in the thickness direction, thereby preventing interference between the male helicoid which is engaged with the female helicoid and the cam groove.
For the depth of the cam groove, a certain depth is needed to realize smooth driving and prevent a cam follower from coming off. Therefore, in the case of the cam frame as shown in FIG. 6, the thickness of the cam frame tends to be high.
On the other hand, in order to respond to recent demands for a smaller size of camera, it is absolutely necessary to realize a reduction in size of a lens frame. In order to realize the miniaturization of the lens frame, it is necessary to reduce the thickness of the lens frame.
However, in the zoom lens barrel having the cam frame as shown in FIG. 6, when the thickness is reduced, either one of the depth of the second-lens-group cam groove 50c and the depth of the female helicoid 51, or both of them, have to be reduced. Accordingly, there is the following problem: the amount of engagement of the cam and that of the helicoid are reduced. For example, if an external force of any impact or blow is applied to the zoom lens barrel, the strength is not maintained.
As this kind of conventional zoom lens barrel, there is a zoom lens barrel proposed in, for example, Japanese Unexamined Patent Application Publication No. 10-293239 which intends to miniaturize a lens frame (cam frame).
In the zoom lens barrel according to the proposal, on the inner surface of a cam frame which is rotated, segments of a female helicoid which is engaged with a male helicoid formed on the outer surface of one lens support cylinder, and cam grooves in which follower pins protruded on another lens frame are fitted are formed so that at least one part of the female helicoid is overlapped with at least one part of the cam grooves in the axial direction. The zoom lens barrel is characterized in that areas in each of which the segment of the female helicoid is formed and areas in each of which the cam groove is formed are separately provided on the cam frame so that the cam grooves do not cross the female helicoid.
According to the proposal, the segments of the female helicoid and the cam grooves can be formed without shifting the positions of the segments of the female helicoid and the cam grooves longitudinally in the thickness of the cam frame. Thus, while the depth of the female helicoid and the depth of each cam groove are being maintained, the thickness of the cam frame can be reduced.
However, in the zoom lens barrel according to the proposal, since the segments of the female helicoid and the cam grooves are alternately formed in the circumferential direction on the inner surface of the cam frame, the whole area where the female helicoid is formed is small. Consequently, since the area where helicoid engagement is achieved in the circumferential direction is reduced, it is difficult to maintain enough strength. Further, the zoom lens barrel is generally required to have light shielding efficiency. The helicoid engagement can improve the light shielding efficiency. However, the light shielding efficiency accomplished by the helicoid engagement cannot be utilized because there are portions in each of which the helicoid engagement is not performed in the circumferential direction as mentioned above.
On the other hand, since driving loads on the zoom lens barrel are reduced to realize smooth driving, a reduction in lead angle is desired. Accordingly, a certain area is needed in the circumferential direction to such an extent that a desired amount of movement is ensured while the small lead angle is being maintained. However, since the female-helicoid forming areas and the cam-groove forming areas are separately formed in the circumferential direction on the inner surface of the cam frame, both of each female-helicoid forming area and each cam-groove forming area are small. Thus, it is difficult to miniaturize the zoom lens barrel while the driving loads on the zoom lens barrel are being reduced.
As mentioned above, in the conventional zoom lens barrel shown in FIG. 6, when the thickness of the cam frame is reduced in order to reduce the size of the cam frame, either one of the depth of the second-lens-group cam groove 50c and the depth of the female helicoid 51, or both of them, have to be reduced. Consequently, the amount of engagement of the cam and that of the helicoids are deteriorated. There is the following problem: for instance, if an external force of any impact or blow is applied to the zoom lens barrel, the strength is not maintained.
In the conventional zoom lens barrel disclosed in Japanese Unexamined Patent Application Publication No. 10-293239, since the segments of the female helicoid and the cam grooves are alternately formed in the circumferential direction on the inner surface of the cam frame, the area of the helicoid engagement in the circumferential direction is reduced. Thus, there is such a problem that it is difficult to obtain enough strength. There is also such a problem that the light shielding efficiency may be degraded in the portions where the helicoid engagement is not performed.
It is an object of the present invention to provide a lens barrel in which when an external force of any impact or blow is applied to the zoom lens barrel, strong coupling of lens frames can be maintained and a small size of each lens frame can be realized.
Briefly, according to the present invention, the lens barrel comprises: a first frame member having on an inner surface thereof a female helicoid and cam grooves each having substantially a same lead angle as a lead angle of the female helicoid, said cam groove being deeper than the female helicoid; and a second frame member which is placed inside the first frame member and which includes a male helicoid engaged with the female helicoid, and a cam follower engaged with the cam groove, wherein the first frame member is rotatable relative to the second frame member, so as to cause one of a forward and backward movement of the second frame member.